Phoma Stem Canker

Pathogen

Fungus

Hosts

Oilseed Rape and other susceptible cruciferous weeds and crops

Symptoms

Phoma leaf spot infection can begin from September as crops emerge. The initial symptoms are white to fawn circular lesions which become dotted with small black fruiting bodies. These leaf lesions are green underneath. Occasionally they cause partial leaf death before winter, but generally have a minimal effect on growth until spring.

These large leaf lesions are caused by Leptosphaeria maculans (Phoma A). A second species, Leptosphaeria biglobosa (Phoma B) produces small dark lesions with few fruiting bodies. L. biglobosa can spread to stems but has less impact than L. maculans on yield. Subsequently the fungus grows from the leaf spot to the stem via the petiole. The rate of this growth varies widely; from 5mm per day at 20°C down to 1mm per day at 5°C.

In summer these stem cankers cause lodging and premature ripening.

Development

In the UK the principle source of initial infection is air-borne ascospores released from the stubble or debris of a previous oilseed rape crop in proximity to a new crop during warm, humid weather. They are dispersed by air currents and land on and infect the leaves, and sometime root collars, of young plants where lesions develop. Early epidemics are associated with above average rainfall in August and September. If the cotyledons are infected, seedlings may die in the autumn

Sunken brown stem canker symptoms appear around six months after initial infection. They gradually enlarge, girdle the stem and weaken it leading to early ripening, lodging and plant death. There may be some spread to pods which develop brown lesions with a black margin, potentially leading to premature ripening and seed infection which can be a secondary source of infection spreading to new crops. Although a second type of spore (pycnidiospore) is produced on leaf lesions, these are considered to be of minor importance under UK conditions.

Favourable Factors

High temperatures and high rainfall.

Importance

Phoma is the main serious autumn disease problem for oilseed rape and later sown crops are at greatest risk. The pathogen's impact is more noticeable in the following summer when stem canker develops and plants ripen prematurely or lodge where stems are weakened.

Typical yield losses in unprotected crops of susceptible varieties are in the range of 0.5 to 0.7 t/ha.

Control

• Varietal resistance as indicated on the recommended list, the implications of a resistant variety are that you can correctly time fungicide applications in the autumn.

• An active seed treatment can reduce cotyledon and early leaf infections caused by seed-borne Phoma infection.

• Early drilling can be helpful, as the fungus will take longer to reach the stem of advanced plants with larger leaves

• Effective volunteer rape and cruciferous weed control will reduce the sources of infection

• Foliar fungicides. In the autumn the threshold is reached when 10 – 20 % of plants are showing leaf symptoms – The 10 % guide is for smaller plants (before the 6 true leaf stage) and 20 % for larger plants.

• Thorough cultivation that buries the stubble of previous oilseed rape crops in close proximity to new crops and any resting spores in the soil can greatly reduce its impact.